The present invention relates generally to metal rolling mills and more particularly to a scheme for controlling workpiece edge taper or "feather" in rolled metal workpieces, hereinafter also referred to as strip. In this specification the term "edge taper" and "feather" are interchangeably used.
In the discipline of metal rolling, it has been long been known that control of the transverse thickness profile on the final rolling pass is necessary to limit overweight, and that control of thickness profile on successive passes is essential in producing strip of acceptable flatness. The difference in strip thickness at edge and center is referred to as strip crown, and the crown and flatness characteristics combined are often referred to as the strip "shape".
In the prior art, crown has been defined in terms of strip thickness profile over a region excluding the outermost 40-50 mm at each edge. For example, Wilmotte et al., in "A New Approach to the Computer Setup of a Hot Strip Mill", Iron and Steel Engineer, September, 1977 (p. 70) exclude the outermost 40 mm at each edge before defining strip profile indices. There are two primary reasons for this exclusion of the tapered edge regions in prior art considerations. First, most producers outside of the United States of America continue to sell hot rolled strip by actual weight rather than by Theoretical Minimum Weight (TWM) as is the practice in the United States. This reduces the importance of the strip overweight problem and, thus, factors which influence overweight. Secondly, the analysis of the deformation of rolls and strip in the edge regions is complicated by many factors. For example, the strip entering the final pass already exhibits edge taper as well as unknown temperature profile in the extreme edge region. The roll, which is generally composed of shell and core sections of different materials, recovers from its deformed to its undeformed state at strip edge in a manner not previously examined in the rolling technology. And, finally, the flow characteristics of the strip as it changes from a constrained environment over most of its width to an unconstrained environment at its extreme edges defy exact analysis. The result of these circumstances has been the neglect of the strip edge behavior even though, as will be shown, it is a significant factor in strip overweight.
Prior art shape control has addressed the control of strip crown and flatness through load distribution; i.e., the force and draft on successive passes through the mill stand or succession of mill stands. A key factor influencing strip crown is the unloaded roll crown. In simpler systems, roll crown is governed by the roll grinding practice, the thermal expansion and wear of the rolls in the mill stand. In more complex systems such as those having roll bending systems, of which more will be said later, the effect of the roll bending system is also considered in estimating the unloaded roll crown. The strip crown produced in passing through the mill rolls is determined by the unloaded roll crown and the deflection of the mill rolls by the rolling force. Thus, a given roll crown and a given delivered strip crown will determine a corresponding rolling force. The draft required to produce that force can be determined from the deformation resistance of the strip.
Examples of workpiece shape control which consider force and draft as well as roll crown to control the strip crown and shape are found in the previously cited Wilmotte article and in U.S. Pat. No. 3,630,055 "Workpiece Shape Control" by Donald J. Fapiano et al., issued Dec. 28, 1971 and its improvement U.S. Pat. No. 4,137,741 "Workpiece Shape Control" issued to Donald J. Fapiano et al., on Dec. 22, 1977. Neither of these patents includes roll bending as a means of controlling roll crown and both assume a specified strip crown. The aspects of roll bending to control crown have, however, been known for a long period of time and an example of such a system and its effects is found in the article "Theory and Practical Aspects in Crown Control" by Dr. M. D. Stone and R. Gray which article was published in Iron and Steel Engineering Yearbook, 1965. This article, as well as the foregoing patents and article are specifically incorporated hereinto by reference for their teachings.
While various aspects of crown control and the shape control have been known for years, what has not been previously understood is that the above procedures also determine, to a large extent, the resulting strip edge taper or feather. These terms refer to the abrupt reduction in strip thickness which occurs in the region of from one to two inches from the edge of the strip. This change in thickness can be as much as 0.01 inch or more and often exceeds 0.005 inch. Although the strip normally has its sides trimmed, this trimming usually amounts to only .cuberoot. to 1/2 of an inch, in steel applications, and thus considerable feather can remain, even after trimming. Underwriters Laboratories' standards specify that the edge should be measured at least 3/8-inch (10 mm) from a cut edge and at least 3/4-inch (20 mm) from the mill edge. Thus, if the feather is severe, the gage targets must be adjusted upwardly to avoid undersize edges with resultant strip overweight.